Measuring instrument for version angle

ABSTRACT

An instrument for measuring an angle about a cavity axis defined by a cavity extending into a head of a bone is provided, the bone having been resected on a plane that is inclined relative to the cavity axis. The instrument includes a stem arranged to be inserted into the cavity, a plane finder and a scale. The plane finder is coupled to the stem and arranged to contact the resection surface as the stem is inserted into the cavity and to move relative to the stem until the plane finder lies in a plane parallel to the resection surface. Movement of the plane finder relative to the stem is indicative of the angle about the cavity axis between a radial position indicated by part of the stem and a line extending perpendicularly from the resection surface. The scale is configured such that movement of the plane finder relative to the stem is indicated by the position of a portion of the plane finder relative to the scale.

FIELD OF THE INVENTION

The present invention relates to a surgical instrument. In particular,the present invention relates to a surgical instrument used during animplantation procedure for a reverse shoulder prosthesis. Moreparticularly, the present invention relates to an instrument formeasuring an angle about a cavity axis defined by a cavity extendinginto a head of a bone. The present invention also relates to a method ofmeasuring an angle about a cavity axis defined by a cavity extendinginto a head of a bone.

BACKGROUND OF THE INVENTION

A humerus-scapular joint (referred to herein as a shoulder joint)prosthesis comprises a humeral component having a stem part which can befitted into a reamed cavity within the medullary canal of the humerus,and a glenoid component for attachment to the glenoid. The humeralcomponent and the glenoid component comprise corresponding bearingsurfaces which articulate together as the joint moves. In a naturalshoulder joint the humeral component comprises a convex head, whicharticulates against a concave bearing surface on the glenoid. Thisstructure is reproduced in an “anatomic” shoulder joint prosthesis, inwhich the humeral component includes a stem part and a head part with aconvex bearing surface and the glenoid component provides a concavebearing surface. The stem part is implanted within the humerus. The headpart is fitted to the stem part, or is formed integrally with the stempart, so that it sits above a resection surface of the humerus. Anatomicprostheses are suitable for implantation in patients where joint tissuehas degraded (for example, due to arthritis).

The structure of the anatomic joint is reversed in a “reverse” shoulderjoint prosthesis. The glenoid component includes a convex head, and thehumeral component has a concave recess in the epiphysis, in which thehead on the glenoid component can be received and articulate.

The humeral component of a reverse joint prosthesis, including theepiphysis part which provides the bearing surface, may be implantedalmost entirely within the humerus.

The biomechanical properties of the patient's joint are altered when areverse shoulder joint prosthesis is implanted because the center ofrotation of the joint is shifted medially. A reverse shoulder jointprosthesis is suitable for implantation in a patient with damaged cuffmuscle tissue. The shift of the center of rotation allows manipulationof the arm using the deltoid muscle because of the increased mechanicaladvantage.

A reverse shoulder prosthesis is described in WO-2007/039820 (DePuy(Ireland) Ltd). Such a joint prosthesis is available commercially andsold by DePuy Products Inc. under the trade name Delta Xtend.

The process of implanting a reverse shoulder prosthesis involves the useof a range of surgical instruments. Embodiments of the present inventionrelate to improved surgical instruments for use in such an implantationprocedure.

It is known to provide a humeral component of a reverse shoulderprosthesis comprising a stem and an epiphyseal cup. The stem and the cupmay be provided as separate components. It is important to ensure thatduring implantation the humeral component is accurately aligned with thecavity formed within the end of the humerus to ensure that theprosthesis is not misaligned.

It is an objection of embodiments of the prior art to obviate ormitigate one or more of the problems of the prior art, whetheridentified herein or elsewhere.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is providedan instrument for measuring an angle about a cavity axis defined by acavity extending into a head of a bone, the bone having been resected ona plane that is inclined relative to the cavity axis, the instrumentcomprising: a stem arranged to be inserted into the cavity; a planefinder coupled to the stem arranged to contact the resection surface asthe stem is inserted into the cavity and to move relative to the stemuntil the plane finder lies in a plane parallel to the resectionsurface, movement of the plane finder relative to the stem beingindicative of the angle about the cavity axis between a radial positionindicated by part of the stem and a line extending perpendicularly fromthe resection surface; and a scale configured such that movement of theplane finder relative to the stem is indicated by the position of aportion of the plane finder relative to the scale.

The plane finder may be coupled to the stem via a handle portion, theangle about the cavity axis being indicated by movement of the planefinder relative to the handle.

The stem may comprise a broach configured to form an eccentric enlargedportion of the cavity at a proximal portion of the cavity. The planefinder may be coupled to the stem via a handle portion, the handlefurther comprising an impaction surface arranged such that an impactionforce applied to the impaction surface is transmitted to the broach todrive the broach into the cavity. The broach may comprise anterior andposterior fins for engaging the sidewalls of the cavity and a medial finfor forming the eccentric enlarged portion at a medial side of theproximal end of the cavity. The radial position indicated by the stemmay comprise the anterior fin.

The stem may further comprise an eccentric protrusion arranged to locatewithin an eccentric enlarged portion of the cavity at a proximal portionof the cavity to determine the rotational position of the stem withinthe cavity.

The plane finder may comprise an elongate rocker bar pivotably coupledto the stem such that the rocker bar pivots about a pivot axis extendingtransverse to the axis of the stem. The rocker bar may be configured topivot about its pivot axis until it contacts the resection surface oneither side of the pivot axis. The rocker bar may be configured toprevent further insertion of the stem into the cavity when the rockerbar contacts the resection surface on either side of the pivot axis. Thepivot axis may lie in a plane that, in use, is parallel to the resectionsurface, the pivot axis intersecting both the axis of the stem and aline extending from the cavity axis perpendicular to the resectionsurface.

The instrument may further comprise a yoke coupled to the stem such thatthe yoke is slidable in the pivot plane of the rocker bar, the yokecomprising a pair of arms, each arm extending towards the rocker bar onopposite sides of the pivot point, wherein pivoting of the rocker barcauses the rocker bar to contact an arm of the yoke causing the yoke toslide away from the rocker bar. Movement of the plane finder relative tothe stem may cause the yoke to slide, the yoke translating movement ofthe plane finder to the scale such that movement of the plane finderrelative to the stem is indicated by the position of a portion of theyoke relative to the scale. The scale may be arranged such that movementof the yoke can be measured by comparing the position of a fixed pointon the yoke to the scale.

The plane finder may comprise a plate coupled to the stem, the plateincluding a surface defined by a plane which intersects the axis of thestem at the same angle as that at which the resection surface of thebone intersects the cavity axis, the plate being configured to rotateabout the axis of the stem until the surface of the plate lies in aplane parallel to the resection surface. The surface of the plate maylie in a plane parallel to the resection surface and is in contact withthe resection surface, further insertion of the stem into the cavity isprevented.

According to a second aspect of the present invention there is provideda method of measuring an angle about a longitudinal axis defined by acavity extending into a head of a bone, the bone having been resected ona plane which is inclined to the longitudinal cavity axis, the methodcomprising: inserting a stem into the cavity; coupling a plane finder tothe stem, the plane finder contacting the resection surface as the stemis inserted into the cavity, the plane finder moving relative to thestem until the plane finder lies in a plane parallel to the resectionsurface; and measuring movement of the plane finder relative to the stemat a scale coupled to the stem, said measured movement being indicativeof the angle about the longitudinal cavity axis between a radialposition indicated by part of the stem and a line extendingperpendicularly from the resection surface.

The stem may comprise a broach and the plane finder may be coupled tothe stem via a handle portion, the handle further comprising animpaction surface. Said step of inserting the stem into the cavity mayfurther comprise applying an impaction force to the impaction surfacesuch that the impaction force is transmitted to the broach driving thebroach into the cavity forming a medial enlarged portion of the cavityat a proximal portion of the cavity.

The plane finder may comprise an elongate rocker bar pivotably coupledto the stem, said step of measuring movement of the plane finderrelative to the stem further comprising measuring the pivoting of therocker bar about a pivot axis extending transverse to the axis of thestem when the rocker bar contacts the resection surface on either sideof the pivot axis preventing further insertion of the stem into thecavity.

Said step of inserting the stem into the cavity may further compriseinserting the broach into a cavity in the end of a humerus such that thebroach is orientated about the longitudinal cavity axis in an anatomicposition by aligning an anterior rib with the anterior aspect of thebicipital groove.

BRIEF DESCRIPTION OF THE INVENTION

The present invention will now be described, by way of example only,with reference to the following figures, in which:

FIG. 1 illustrates in an exploded view a cutting guide for guiding acutting tool used to resect the head of a humerus, the cutting guidebeing arranged to be used when a superior-lateral surgical approachexposes the humerus;

FIG. 2 illustrates in an exploded view a cutting guide for guiding acutting tool used to resect the head of a humerus, the cutting guidebeing arranged to be used when a deltoid-pectoral surgical approachexposes the humerus;

FIG. 3 illustrates the cutting guide of FIG. 1 assembled and in positionon the head of a humerus, and illustrating the use of an alignment pinfor determining the orientation of the cutting guide;

FIG. 4 illustrates the cutting guide of FIG. 1 assembled and in positionon the head of a humerus;

FIG. 5 illustrates the cutting guide of FIG. 2 assembled and in positionon the head of a humerus;

FIGS. 6 and 8 illustrate a cutting plate forming part of the cuttingguide of FIG. 1 attached to the head of a humerus in first and secondconfigurations;

FIGS. 7 and 9 illustrate a cutting plate forming part of the cuttingguide of FIG. 2 attached to the head of a humerus in first and secondconfigurations;

FIG. 10 illustrates a first reamer being used to ream portions of aglenoid such that a glenoid component of a reverse shoulder prosthesiscan be attached to the glenoid;

FIG. 11 illustrates a second reamer being used to ream portions of aglenoid such that a glenoid component of a reverse shoulder prosthesiscan be attached to the glenoid;

FIG. 12 illustrates the reamer of FIG. 11 in a side view positionedagainst the reamed glenoid;

FIG. 13 illustrates an intramedullary reaming guide and an alignmentinstrument for aligning the intramedullary reaming guide relative to aresected humeral head when the intramedullary reaming guide is insertedinto a cavity reamed in the medullary canal of the humerus;

FIG. 14 illustrates the intramedullary reaming guide and the alignmentinstrument of FIG. 13 coupled together;

FIGS. 15 and 16 illustrate the intramedullary reaming guide and thealignment instrument of FIG. 13 during implantation of theintramedullary reaming guide into a cavity reamed in the medullary canalof the humerus;

FIG. 17 illustrates the intramedullary reaming guide of FIG. 13implanted into a cavity reamed in the medullary canal of the humerus;

FIG. 18 illustrates a centered reaming adapter coupled to aintramedullary reaming guide implanted into a cavity reamed in themedullary canal of the humerus as illustrated in FIG. 17;

FIGS. 19 and 20 illustrate first and second sizing guides respectivelycoupled to the centered reaming adapter illustrated in FIG. 18;

FIG. 21 illustrates a first eccentric reaming adapter coupled to aintramedullary reaming guide implanted into a cavity reamed in themedullary canal of the humerus as illustrated in FIG. 17;

FIG. 22 illustrates a first sizing guide coupled to the eccentricreaming adapter illustrated in FIG. 21;

FIG. 23 illustrates a second sizing guide coupled to a second eccentricreaming adapter illustrated, which in turn is coupled to aintramedullary reaming guide implanted into a cavity reamed in themedullary canal of the humerus as illustrated in FIG. 17;

FIG. 24 illustrates a reaming head being used to ream an epiphysiscavity in a resected humeral head;

FIG. 25 illustrates a broach and a broach insertion instrument beingused to enlarge a reamed cavity within the medullary canal of a resectedhumeral head;

FIG. 26 illustrates portions of the broach and broach insertioninstrument of FIG. 25 being used to measure a rotational offset betweenthe rotational position of the broach and a line which extends normal toa resection surface and intersects a longitudinal axis of the humerusdefined by the reamed cavity;

FIG. 27 illustrates part of a modular humeral component of a reverseshoulder prosthesis;

FIG. 28 illustrates part of a convex bearing head forming part of areverse shoulder prosthesis and part of a convex bearing headorientation guide for correctly aligning the eccentricity of the convexbearing head;

FIG. 29 illustrates the convex bearing head and convex bearing headorientation guide of FIG. 28 during attachment of the convex bearinghead to a glenoid;

FIG. 30 illustrates the orientation guide of FIG. 29 in cross sectionalong its longitudinal axis during attachment of the convex bearing headto a glenoid, including a screwdriver passing through a central bore ofthe orientation guide;

FIG. 31 illustrates a front view of the convex bearing head of FIG. 28;

FIG. 32 illustrates a reaming guide coupled to the epiphysis of thehumeral component of a reverse shoulder prosthesis for use within arevision procedure to remove cortical bone around the epiphysis;

FIG. 33 illustrates a reaming head coupled to the reaming guide of FIG.32; and

FIG. 34 illustrates a humeral head implant couple to a humeral headafter reaming using the reaming head of FIG. 33.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

A reverse shoulder prosthesis may be of the form available commerciallyand sold by DePuy Products Inc. under the trade name Delta Xtend ReverseShoulder System. Such a reverse shoulder system particularly suitablefor treating shoulder cuff tear arthropathy. The normal biomechanics ofa patient's scapula and humeral components are reversed. Advantageously,the gleno-humeral joint center of rotation is moved medially andinferiorly increasing the deltoid lever arm and the deltoid tension thusallowing the muscles of the deltoid group to compensate for rotator cuffdeficiency.

A reverse shoulder prosthesis comprises two primary components: ahumeral component implanted into a reamed cavity within the medullarycanal of a resected humeral head and a glenoid component attached to areamed portion of the glenoid part of the scapula. The humeral componentmay either comprise a modular humeral stem part and an epiphysis part ora single integral component comprising both a humeral stem and anepiphysis. The modular humeral component is preferably designed to forma press fit in a reamed humeral cavity. The integral humeral componentis preferably designed to be cemented in position. For press fit humeralcomponents the surface of the implant may be coated with a materialwhich encourages bone in growth thereby securing the implant inposition, for instance hydroxyapatite (HA) coated titanium alloy. Theglenoid component may be secured primarily by screws into the glenoidwith a HA coating for secondary fixation.

The stem part of the humeral component may be similar in form to thestem part of an anatomic shoulder prosthesis. For a modular humeralcomponent it is known for the epiphysis part to be either centered uponthe humeral component or offset in a posterior direction to allow foradjustable retroversion, thereby allowing for increased internalrotation of the joint. The plane of the upper face of the epiphysis partis typically at 155° to the axis of the stem part, which increases thestability of the implanted prosthesis.

The glenoid component comprises a mounting plate (alternatively referredto as a metaglene) arranged to be attached to a reamed portion of theglenoid and a convex bearing head (alternatively referred to as aglenosphere) comprising a convex bearing surface mountable upon themounting plate. The convex bearing head comprises part of a sphere. Theconvex bearing head may be eccentric (that is, having a fixation holethat is not positioned at the centre of the bearing surface of theconvex bearing head) in order to increase the range of motion of theshoulder prosthesis and reduce the risk of scapular erosion.

Between the humeral component and the glenoid component there isprovided a humeral cup formed from a material having a low frictionsurface, such as polyethylene, in order to maximize the range of motionof the shoulder prosthesis and reduce the risk of scapular erosion. Thehumeral cup is typically coupled to the epiphysis.

In the event of problems arising within implanted reverse shoulderprostheses a reverse shoulder prosthesis can be converted to ananatomical prosthesis. To achieve this, the convex bearing head and themounting plate are removed from the glenoid and the humeral cup isremoved from the epiphysis. A convex bearing head may then be attachedto the epiphysis, arranged to articulate against the glenoid and theacromion.

A surgical procedure for implanting a reverse shoulder prosthesis andoptionally converting the prosthesis to an anatomic prosthesis, andparticular the surgical instruments used in such a procedure will now bedescribed.

Prior to surgery an initial assessment is made of the humerus and theglenoid using radiographic and CT imaging to determine whether there issufficient bone stock for implantation of the humeral component and theglenoid component. If the patient is suitable for treatment, then theimaging may be measured in order to determine the appropriate size ofimplants, though the final decision is typically left to the surgeon'sdiscretion.

A reverse shoulder prosthesis may be implanted using a surgical approachinvolving either a superior-lateral incision or a deltoid-pectoralincision. The decision is subject to the surgeon's preference andclinical parameters. The chosen approach affects the surgicalinstruments and techniques used, in particular the instruments used forresecting the humeral head, as will be described in greater detailbelow.

A superior-lateral approach comprises forming an incision eitheranterior-posterior along the lateral edge of the acromion or in alateral direction starting from a superior position on the shoulder. Theshoulder is dissected until the humeral head is visible at the anterioredge of the acromion. The arm may then be externally rotated and thehead dislocated antero-superiorly to facilitate positioning of a cuttingguide. The superior-lateral approach allows for a clear view of theglenoid and therefore facilitates the implantation of the glenoidimplant components, in particular when the glenoid is retroverted.

A deltoid-pectoral approach comprises forming an incision from themidpoint of the clavicle to the midpoint of the arm. The shoulder isdissected until the humeral head is visible and can be dislocated. Thedeltoid-pectoral approach has the advantage of offering an enhanced viewof the inferior part of the glenoid. If revision surgery is required inorder to convert the humerus-scapula joint to an anatomicalconfiguration the deltoid-pectoral approach is preferred as it allowsfor a longer humeral incision.

Regardless of the surgical approach, once the humeral head is visibleand has been dislocated the first step is to form an intramedullarycavity. The cavity runs from the humeral head parallel to thelongitudinal axis of the humerus. The cavity defines a longitudinal axisextending along the cavity into the humerus. A pilot hole must first bedrilled into the humeral head, passing directly down into the medullarycanal along the bone. A series of hand reamers having progressivelylarger diameters are then used to enlarge the cavity until there iscontact with cortical bone of the intramedullary canal of the humerus.The diameter of the final reamer used determines the size of the cuttingguide assembly support rod, intramedullary reaming guide and the finalhumeral component, as will be described herein. For example, if a 12 mmreamer begins to gain purchase in the intramedullary cortical bone (andso is the largest reamer used) then a 12 mm stem for the humeralcomponent will be required.

Once the intramedullary cavity has been formed, then the humeral headcan be resected. Referring to FIGS. 1 and 2 these respectivelyillustrate in exploded views alternative cutting guide assemblies thatmay be used to guide a cutting tool for resecting the humeral head. Thecutting guide assembly illustrated in FIG. 1, generally referred to asreference numeral 1, is arranged to be used when the surgical approachis superior-lateral, whereas the cutting guide assembly illustrated inFIG. 2, generally referred to as reference numeral 1 a , is arranged tobe used when the surgical approach is deltoid-pectoral.

The required resection of the humeral head is the same regardless of thesurgical approach. The resection surface is typically required to be atan angle of 155° to the longitudinal axis of the humerus defined by theintramedullary cavity. The cutting guide assemblies 1, 1 a illustratedin FIGS. 1 and 2 present a cutting surface on a cutting plate that isautomatically orientated at 155° to the longitudinal axis of thehumerus. The resection surface faces medially and superiorly; that is,it faces towards the glenoid. For a superior-lateral approach thevisible portion of the humeral head is predominantly the superior andlateral portions of the humeral head, whereas for a deltoid-pectoralapproach it is predominantly the anterior portion of the humeral headthat is visible. Consequently, a separate cutting guide assembly isrequired for each surgical approach. Each cutting assembly is suitablefor performing surgical procedures on either a patient's left or rightarm. The superior-lateral cutting guide assembly of FIG. 1 may be usedfor either the right arm (as illustrated) or the left arm by simplyrotating the whole assembly. The deltoid-pectoral cutting guide of FIG.2 may be used for either the right arm (as illustrated) or the left armby rotating the cutting plate 4. As shown in FIG. 2, the cutting plate 4is engraved with “RIGHT” on a first side indicating that it isorientated for used on a right shoulder and “LEFT” on a second oppositeside (not visible) indicating that it is orientated for use on a leftshoulder. It will be apparent from the description below that thecutting guide assemblies 1, 1 a comprise a selection of modularcomponents, some of which are common to each surgical approach. FIGS. 1and 2 illustrate cutting guide assemblies 1, 1 a suitable for resectingthe head of a right humerus of a patient.

Each cutting guide assembly 1, 1 a comprises a cutting plate 2, 4illustrated in FIGS. 1 and 2 respectively. Each cutting plate comprisesa cutting surface that defines the resection surface, and may bemaneuvered until it is in the optimal position for performing theresection. The resection is then achieved by the surgeon aligning acutting tool with the cutting surface such that the resection surface isparallel to the cutting surface of the cutting plate.

The cutting guide assemblies 1, 1 a illustrated in FIGS. 1 and 2 eachcomprise the same support rod 6. The support rod 6 comprises an elongaterod including a first portion 8 for insertion into the reamedintramedullary cavity. Each cutting guide assembly is provided with arange of support rods 6, with each support rod 6 being provided with adifferent diameter first portion 8 corresponding to the differingdiameters of the reamers used to form the intramedullary cavity. Forinstance, if a 12 mm intramedullary cavity has been formed then a 12 mmdiameter support rod 6 must be used to ensure that the support rod 6forms a close fit in the intramedullary cavity.

Each support rod 6 comprises a flange 10 which forms a depth stoppreventing over insertion of the support rod 6 into the intramedullarycavity by coming to rest against the top surface of the humeral head.Adjacent to the flange 10 the support rod 6 further comprises areference formation 12. The reference formation 12 is formed as a rib.The orientation of the reference formation 12 relative to thelongitudinal axis of the humerus determines the orientation of theresection surface about the longitudinal axis of the humerus. Thesupport rod 6 further comprises a T shaped handle 14 which may bemanipulated by a surgeon in order to rotate the support rod 6 within theintramedullary cavity to adjust the orientation of the resectionsurface, as will be described in greater detail below.

Once the support rod 6 has been fully inserted into the intramedullarycavity the remainder of the cutting guide assembly may be assembled. Thecutting guide assemblies 1, 1 a illustrated in FIGS. 1 and 2 eachcomprise a separate cutting plate mount 16, 18 respectively for couplingthe cutting plate 2, 4 to the support rod 6. Each cutting plate mount16, 18 comprises a clamp formed as a collar 20 arranged to surround theshaft of the support rod 6 and engage the reference formation 12. Thecollar 20 incorporates a groove 22 arranged such that when coupled torib 12, collar 20 is prevented from rotating about the support rod 6. Alocking screw 24 is provided, which passes through a corresponding holein the collar 20 and engages the shaft of the support rod 6 holding thecollar in place.

The purpose of cutting plate mounts 16, 18 is to position the cuttingplates 2, 4 in an appropriate position to define the plane of theresection surface. Consequently, each cutting plate mount 16, 18 furthercomprises a shaft 26, 28 that extends from the collar 20. Shafts 26, 28extend from collar 20 along an axis parallel to the plane of the cuttingsurface of the cutting plate 2, 4 (and hence parallel to the resultingresection surface). For the cutting guide assembly illustrated in FIG. 1when assembled for a superior-lateral approach of a right shoulder theshaft 26 extends from the collar 20 laterally so as to protrude from thepatient's shoulder through the incision. Consequently, in order to lieparallel to the resection surface, shaft 26 extends superiorly andlaterally. For the cutting guide illustrated in FIG. 2 when assembledfor a deltoid-pectoral approach of a right shoulder the shaft 28 extendsfrom the collar 20 anterially so as to protrude from the patient'sshoulder through the incision. Consequently, in order to lie parallel tothe resection surface, shaft 28 extends perpendicularly from the supportrod 6.

Cutting plate 2 illustrated in FIG. 1 is provided with a concave curvededge 30 on the side that in use will be adjacent to the humeral head.Curved edge 30 is intended to reflect the profile of the humeral head soas to allow the cutting plate 2 to be positioned close to the lateralportion of the humeral head. Conversely, cutting plate 4 illustrated inFIG. 2 has a straight edge 32 on the side that in use will be adjacentto the anterior portion of the humeral head. Extending superiorly fromeach cutting plate 2, 4 and parallel to the axis of the support rod 6 isa post 34, 36. Posts 34, 36 are slidably received within a respectiveclamp 38, 40, which in turn is slidably mounted upon shafts 26, 28respectively. The position of the clamps 38, 40 with respect to posts34, 36 can be locked by tightening screws 42, 44 in order to preservethe height adjustment of the cutting plate 2, 4 selected by the surgeon.Clamps 38, 40 remain free to slide along shafts 26, 28 so that thecutting plate 2, 4 can be slid close to the humerus before being securedin position (as described below).

The arrangement of the cutting plate mounts 16, 18 is such that for eachcutting guide assembly the cutting plates 2, 4 may be raised or loweredparallel to the longitudinal axis of support rod 6 by sliding posts 34,36 through clamps 38, 40. This allows the surgeon to select theappropriate position of the resection surface along the longitudinalaxis of the humerus. Posts 34, 36 are provided with color codedmarkings, comprising a central red marking 46 and outer green markings48. Normally, the post 34, 36 will be locked in position by therespective clamp 38, 40 such that only the green markings 48 are visibleeither side of the clamp 38, 40. This ensures that the resection surfaceis located along the longitudinal axis of the humerus at the correctposition for most patients (if the support rod 6 is inserted into themedullary canal sufficiently far for the flange 10 to contact the uppersurface of the humeral head). However, on a patient specific basis, thesliding adjustment of posts 34, 36 allows the position of the resectionsurface along the longitudinal axis of the humerus to be adjustedaccording to clinical parameters.

Furthermore, the arrangement of the cutting plate mounts 16, 18 is suchthat for each cutting guide assembly the cutting plates 2, 4 may bebrought closer to, or in contact with, the humeral head by slidingclamps 38, 40 along shafts 26, 28. Given that shafts 26, 28 extendparallel to the required resection surface, once the surgeon hasselected the appropriate level of the resection surface along thelongitudinal axis of the humerus, the cutting plates 2, 4 can be slidtowards the humeral head parallel to the resection surface, such thatthe position of the resection surface is not affected. Bringing thecutting plates into contact with the humeral head advantageously allowsthe surgeon to cut the humeral head by running the cutting tool alongthe cutting surface with less risk of inaccuracy caused by stray motionof the cutting tool.

As illustrated in FIGS. 1 and 2, the cutting surface is defined by thesuperior side 50, 52 of the cutting plates 2, 4. However, as will beappreciated, the arrangement is such that a surgeon would be compelledto cut around posts 34, 36 and also the support rod 6 where it extendsinto the humeral head. As will be described below, once the cuttingplate 2, 4 has been appropriately positioned, the cutting plate 2, 4 canbe locked in position on the humeral head and the support rod 6 and thecutting plate mount 16, 18 removed in order to ease a surgeon's work inresecting the humeral head.

As has been described above, the provision of separate cutting guideassemblies 1, 1 a optimized for use with either a superior-lateral or adeltoid-pectoral surgical approach allows a surgeon to accuratelyposition a cutting plate 2, 4 (and hence the resection surface) at adesired level along the longitudinal axis of the humerus by adjustmentof clamp 38, 40. The desired orientation about the longitudinal axis ofthe humerus is set by rotation of the support rod 6. The correct angleof the resection surface with respect to the longitudinal axis of thehumerus is set automatically by the angle subtended between the cuttingplate 2, 4 and post 34, 36, which in use is parallel to the longitudinalaxis of cavity and hence parallel to the longitudinal axis of the bone.The cutting guide assemblies 1, 1 a allow these parameters of theresection surface to be set in a controlled fashion which is not solelydependent upon the surgeon's skill and judgment in order to correctlyposition the resection surface. The cutting guide assemblies 1, 1 aallow the position of the resection surface to be finely adjusted beforeany cutting step is required.

As noted above, the orientation of the resection surface about thelongitudinal axis of the humerus can be adjusted by rotating the supportrod 6 within the intramedullary cavity. In order to assist the alignmentof the resection surface, an upper portion 54 of the support rod 6further comprises a series of alignment holes 56 which pass through thehandle 6. The alignment holes 56 include a primary alignment hole 58indicated by a flared entrance hole. As can be seen, the axis of theprimary alignment hole is parallel to the axis of the referenceformation 12 defined by the long axis of the rib. The remainingalignment holes 56 form a series of alignment holes extending though thesupport rod 6 at differing radial directions.

When the support rod 6 is inserted into the intramedullary cavity, analignment rod 60 can be inserted into one of the alignment holes 56 anduse to rotationally align the cutting guide handle 6, as is shown inFIG. 3. FIG. 3 shows a cutting guide in accordance with FIG. 1 beingused to locate cutting plate 2 relative to a humeral head 62 when thehumeral head 62 has been exposed using a superior-lateral surgicalapproach. Soft tissue of the patients shoulder is shown held back byretractors 64. It will be appreciated that an alignment rod 60 may beused to align the cutting guide of FIG. 2 in the same way, given thatthe support rod is the same for each cutting guide.

Adjusting the rotational position of the resection surface varies thedegree of retroversion or anteversion (that is the rotational positionabout the longitudinal axis of the humerus of a line which is normal tothe resection surface and intersects the longitudinal axis of thehumerus) applied to the implanted reverse shoulder prosthesis. Theretroversion or anteversion of the final implant position can beassessed by comparing the axis 68 of the alignment rod 60 with thepatient's forearm axis 66. Rotating the support rod 6 within theintramedullary cavity until the alignment rod axis 68 is parallel to thepatient's forearm axis 66 ensures that required degree of retroversionor anteversion is set for the resection surface. If the alignment pin 60is inserted into the primary alignment hole 58 then 0° retroversion isset for the resection surface.

If alternatively one of other alignment holes 56 are used then apredetermined degree of retroversion or anteversion can be provided tothe resection surface. Typically 0-10° retroversion is applied sinceexcessive retroversion can restrict joint mobility, especially internalrotation. However, care must be taken not to damage the subscapularisinsertion by resecting the humeral head 62 with excessive anteversion.

Once the desired degree of retroversion or anteversion has been set, thelevel of the resection surface can be adjusted as discussed above,typically such that only the green markers 48 are visible on post 34.Usually 1-2 mm of the proximal area of the greater tuberosity isresected (at the level of the supraspinatus insertion on an intactshoulder). The cutting plate 2, 4 can then be slid into contact with thehumeral head 62 by adjusting the position of the clamp 38, 40 alongshaft 26, 28. FIGS. 4 and 5 show the cutting guide assemblies 1, 1 aillustrated in FIGS. 1 and 2 respectively assembled and positioned upona humeral head 62 such that the cutting surface 50, 52 is positioneddefining the plane of the chosen resection surface. For clarity, softtissue surrounding the humeral head 62 is not shown.

As noted above, the cutting plate 2, 4 may be secured to the humeralhead 62 such that the remainder of the cutting guide assembly can beremoved, assisting the surgeon in resecting the humeral head 62 bypassing a cutting tool over the cutting surface 50, 52. As shown inFIGS. 4 and 5, each cutting plate 2, 4 further comprises a pair of guideholes 70, 72 respectively at each end of the cutting plate 2, 4. Oncethe cutting plate 2, 4 is correctly positioned, holes may be drilledthrough the guide holes 70, 72 into the cortical bone with a 3.2 mmdrill bit, using the cutting plate 2, 4 as a drill guide. Fixation pins74 may then be passed through the guide holes 70, 72 preserving thealignment of the cutting plate 2, 4 relative to the humeral head 62.

Once fixation pins 74 are in position, the cutting plate mount 16, 18can be removed from the cutting plate 2, 4 by slackening off lockingscrew 42, 44 thereby freeing post 34, 36 which extends from the cuttingplate 2, 4. Slackening off locking screw 24 allows the clamp 20 to belifted parallel to the longitudinal axis of the support rod 6 such thatcutting plate mount 16, 18 is decoupled from the cutting plate 2, 4without disturbing its position relative to the humeral head 62. Thesupport rod 6 can then be released from the intramedullary cavity. Thecutting plate 2, 4 is supported on the humeral head 62 by the fixationpins 74 as shown in FIGS. 6 and 7 in the relative position determinedthrough the above alignment steps. The cutting plate 2, 4 can be furthersecured to the humeral head 62 by passing a third fixation pin 74through a third guide hole 76, 78. The outer pair of guide holes 70, 72are arranged to be parallel to one another such that the cutting plate2, 4 can slide along fixation pins 74 towards and away from the humeralhead parallel to the resection surface 50, 52. The third, central guidehole 76, 78 defines an axis which is divergent from the axes of thefirst two guide holes 70, 72. Consequently, when the third fixation pin74 is inserted the cutting plate 2, 4 is held firmly in position.

As will be appreciated from FIGS. 6 and 7 the surgeon is able to resectthe humeral head 62 by passing a cutting tool parallel to and next tothe cutting surface 50, 52. Removal of the other components of thecutting guide assembly greatly reduces the complexity of the cuttingstep that must be performed by the surgeon. However, post 34, 36extending from the cutting plate is still in the way of the resectionand must be cut around, possibly resulting in a less accurate resection.In order to avoid this obstacle, before the third fixation pin 74 isinserted, the cutting plate 2, 4 can be removed from the initial pair offixation pins 74 by sliding along the axes of the parallel pair offixation pin 74. The cutting plate 2, 4 can then be inverted andreplaced over the fixation pins 74 as shown in FIGS. 8 and 9. Thecutting plate 2, 4 additionally defines a second cutting surface 80, 82,which now faces superiorly and in the plane previously occupied by thefirst cutting surface 50, 52. Second cutting surface 80, 82 is parallelto the first cutting surface 50, 52 and equidistant from the fixationpins 74. Consequently, the second surface 80, 82 is parallel to the samechosen resection surface. Advantageously, there is no post protrudingfrom the second cutting surface 80, 82 allowing for the resection to beperformed as a single cutting action resulting in a more even resection.Again, a third fixation pin 74 can be provided through divergent guidehole 76, 78 securing the cutting plate 2, 4 in position.

It will be appreciated that in alternative embodiments the cutting plateassembly may be varied. In particular, the coupling mechanism to thesupport rod may be modified to provide alternative mechanisms forcoupling the cutting plates such that cutting plates designed fordifferent surgical approaches are aligned to the same desired resectionplane. Similarly, the height adjustment of the cutting plate may bemodified, for instance by coupling to the cutting plate mount via analternative connection. It will be desirable that any alternativecutting guide assembly retains the ability for the cutting plateassembly to be disassembled while the cutting plate remains attached tothe head of the bone.

Once the resection has been performed, the fixation pins 74 and thecutting plate 2, 4 can be removed from the humeral head 62. A humeralresection protecting plate can be placed over the resected surface inorder to protect the bone from damage during the following surgicalsteps preparing the glenoid.

A forked retractor can be passed under the scapula in order to lever thehumeral head 62 out of the way in order to allow unimpeded access to theglenoid. If the glenoid is not fully visible then a further resection ofthe humeral head 62 may be required. The forked retractor is placedunder the inferior glenoid labrum to move the humerus distally orposteriorly according to the chosen surgical approach (superior-lateralor deltoid-pectoral respectively).

Once the glenoid is fully visible, preparation of the glenoid can begin.Firstly, any remnants of the labrum must be removed from the glenoidface. Additionally, any osteophytes present may also have to be removedto prevent later interference when attaching the mounting plate and theconvex bearing surface to the glenoid.

Particular care is needed when determining the attachment point of themounting plate as this affects the resultant center of rotation of thereverse shoulder prosthesis. The correct mounting plate positionachieves optimal glenoid fixation (that is, the mounting plate is fullyin contact with the glenoid), good range of motion of the shoulder jointand minimal potential for bone impingement (the humeral componentcontacting the scapula around the convex bearing head). Ideally, themounting plate should be positioned on the inferior circular portion ofthe glenoid. A mounting plate positioning tool may be used to determinethe optimal mounting plate position. This comprises a generally circularsizing plate including cut outs such that the glenoid surface is visiblethrough the sizing plate mounted upon a positioning handle which can bemanipulated by the surgeon at a point remote from the glenoid. Thepositioning handle couples to the sizing plate at a point eccentric ofthe center of the sizing plate such that the center of the plate isvisible, and couples to the sizing plate along an axis which divergesfrom an axis normal to the plate (for instance 20°) in order to allowfor maximum visibility of the glenoid.

Once the sizing plate is positioned correctly (for instance, such thatits border follows the inferior edge of the glenoid and the sizing plateis parallel to the glenoid face, or with a slight superior tilt) a guidepin is inserted through a guide hole in the center of the sizing plateinto the glenoid. The guide pin is inserted either perpendicularly tothe glenoid or with a slight superior tilt as determined by the positionof the sizing plate. This ensures that an axis defined by the convexbearing head will be either perpendicular to the glenoid or with aslight inferior tilt, thus reducing the risk of scapular notching due tocontact between the humeral epiphysis component and the scapula. Theposition of the guide pin determines the resulting position of themounting plate as further steps preparing the surface of the glenoid areperformed using the guide pin to locate the surgical instruments, aswill be described below. The guide pin comprises a 2.5 mm diameter rodand is inserted 3-4 cm into the glenoid using a power tool. The sizingplate and positioning handle may then be removed by sliding over theguide pin.

The mounting plate comprises a circular disc having a slightly convexrear side to be mounted within a corresponding concave depression reamedon the glenoid surface. In order to prepare the glenoid surface a twostep reaming process is required. In a first reaming step the glenoid isprepared using a powered circular reamer that is arranged to prepare areamed portion of bone that is the same size as the mounting plate. Asshown in FIG. 10 the powered reamer 100 comprises a circular reamingshell driven by a power tool 102. The reaming shell 100 and the powertool 102 are passed over the guide pin indicated by dashed line 104 bysliding a cannulated shaft over the guide pin such that the reamingposition is fixed. The reaming shell 100 may be 27 mm in diameter toream a concave depression on the glenoid surface corresponding to atypical mounting plate.

Although the mounting plate will be seated correctly after the initialreaming step, the convex bearing head to be mounted upon the mountingplate extends outside of the reamed area. In order to avoid conflictbetween the convex bearing head and the superior area of the glenoid itis necessary to ream the superior area of the glenoid outside of thefirst reamed area. As shown in FIGS. 11 and 12, a manually driven reamer108 is used to ream the superior area 110 of the glenoid 106. The reamer108 comprises a cannulated shaft 112 which slides over the guide pinindicated by dashed line 104. This ensures alignment of the secondreaming step with the first reamed area. The reamer 108 comprises aguide portion 114 which is shaped to be received within the first reamedarea. The guide portion 114 has a non-reaming lower surface which isarranged to slidably engage the first reamed portion of the glenoid asthe cannulated shaft 112 is rotated about the guide pin. The guideportion 114 includes cut away portions 116 such that its positionrelative to the first reamed portion of the glenoid 106 can be observed.

Reamer 108 further comprises an eccentric reaming lobe 118 which extendsfrom the guide portion 114 about a portion of the periphery of the guideportion 114. Eccentric reaming lobe 118 has a reaming lower surfacepositioned to engage the superior area of the glenoid 106. The reamingsurface may comprise reaming formations, such as teeth, as is known inthe art. By rotating cannulated shaft 112 about the guide pin whileapplying pressure towards the glenoid 106 the reaming surface of theeccentric reaming lobe 118 is arranged to remove surface portions of theglenoid, until the guide portion 114 is fully seated within thepreviously reamed portion of the bone. Once this is achieved, as shownin side view in FIG. 12, the glenoid surface is fully prepared toreceive the mounting plate and the convex bearing head.

Advantageously, by providing the second reamer as an eccentric reaminglobe, the second reamer is reduced in size compared to a conventionalcircular reamer such as is used in the first reaming step. This allowsthe second reamer to be inserted through a smaller incision that wouldotherwise be the case. For instance, the maximum dimension (that is, thelength) of the eccentric reaming lobe 118 may be approximately the sameas the diameter of the guide portion 114. The radial extent of theeccentric reaming lobe (from the edge of the guide portion 114 to theedge of the eccentric lobe) may be approximately 8 mm, which isapproximately 0.3 times the diameter of the guide portion. Preferablythe maximum length and the maximum radial extent from the guide pin ofthe eccentric reaming lobe is less than the diameter of the guideportion.

As the second reamer is eccentric, it is necessary to manually drive thesecond reamer such that the eccentric reaming lobe 118 can be rotatedback and forth over the superior area of the glenoid in order reduce theimpact on the remainder of the glenoid and surrounding tissue. However,if necessary, the second reamer can be used to remove other portions ofthe glenoid face anteriorly, posteriorly and inferiorly about thecircular reamed mounting plate portion.

Optionally, after the second reaming step has been completed, thepreparation of the glenoid can be checked by passing a glenoid levelchecker over the guide pin. The glenoid level checker comprises a discof the same shape as the mounting plate and an eccentric lobecorresponding to the same amount of bone that is required to be removedfrom the superior area of the glenoid. The glenoid level checkerincludes cut outs so that the surface of the glenoid may be viewed whilechecking the reaming. No space should be visible between the glenoidlevel checker and the glenoid surface if the reaming has been completedcorrectly. If space is visible between the glenoid surface and theglenoid level checker then further reaming with either the first and/orthe second reamer may be required.

It will be appreciated that in alternative embodiments the eccentricreamer may be varied. For instance, it could be modified to be driven bya motor with a reciprocating action such that the eccentric reaming lobeis repeatedly passed over the same portion of the glenoid surface.

After reaming of the glenoid is complete the guide pin is left in placeand used as a drilling guide for drilling a central hole into theglenoid to receive a central pin of the mounting plate. A cannulatedstop drill includes a central cavity to receive the guide pin is used.The cannulated stop drill includes a flange ensuring that the centralhole is not over drilled.

The mounting plate comprises a disc sized and shaped to be received inthe first reamed portion of the glenoid. The mounting plate furtherincludes a central pin corresponding to the central hole drilled in theglenoid. The central pin incorporates a threaded bore for laterattachment of the convex bearing head (as will be described in greaterdetail below). The exterior surface of the central pin is ribbed so asto form a push fit in the central hole. The mounting plate furthercomprises four fixation holes to receive fixing pins passing into theglenoid to secure the implant. Once the central pin is fully received inthe central hole in the glenoid, if necessary the mounting plate may berotated such that the inferior fixation hole is aligned with theinferior pillar of the glenoid. The surface of the mounting platefurther comprises a vertical alignment mark to ensure correctorientation by aligning the vertical alignment mark with the scapularpillar inferiorly and the base of the coracoid process superiorly (thatis, the vertical alignment mark is aligned with the long axis of theglenoid).

The mounting plate may be gently impacted to ensure that the mountingplate pin is fully seated. Screws may then be implanted through thefixation holes to complete the implantation. The screws may be lockingscrews, as are known in the art, and may be such that the angle ofimplantation can be varied to ensure implantation into good bone stock.Alternative, any other suitable form of screw may be used. The mountingplate implantation is then secure and further humeral head preparationcan be carried out.

To ream the resected humeral head so as to create a cavity to receivethe epiphysis component of the humeral implant, it is necessary toinsert an intramedullary reaming guide into the cavity in the reamedmedullary canal. Referring to FIG. 13, the intramedullary reaming guide200 comprises an elongate stem portion 202 which defines a longitudinalaxis, and a neck portion 204, which defines a neck axis inclined to thelongitudinal axis. The intramedullary reaming guide 200 is provided in arange of sizes determined by the diameter of the stem portion 202. Thesize of intramedullary reaming guide chosen is determined by thediameter of the intramedullary cavity reamed into the humeral head 62,as described above. The intramedullary cavity defines a longitudinalaxis, which is parallel to or aligned with the longitudinal axis of thehumerus. Consequently, when the intramedullary reaming guide 200 isinserted into the intramedullary cavity, rotating the stem portion 202rotates the neck portion 204 about the longitudinal axis of the humerus.

The neck portion 204 further comprises a flange 206, such that when theintramedullary reaming guide 200 is fully inserted into theintramedullary cavity, further insertion is prevented by the flange 206.Adjacent to the flange 206 is a reference formation 208, comprising arib. The reference formation 208 serves to ensure that any posterioroffset when reaming the epiphysis cavity is precisely orientatedrelative to the neck portion 204, as will be described in greater detailbelow. The reference formation 208 also allows the intramedullaryreaming guide to be coupled to an alignment instrument, as will bedescribed below.

The stem portion 202 further comprises at least one and preferably tworibs 210 arranged to cut into the cancellous bone around theintramedullary cavity as the intramedullary reaming guide 200 is driveninto the intramedullary cavity. The ribs 210 prevent the fully insertedintramedullary reaming guide from being rotated about the axis of thestem once the intramedullary reaming guide 200 is fully inserted.Therefore, it is essential that the intramedullary reaming guide 200 iscorrectly orientated before being driven into the humerus.

As noted above, the neck portion 204 defines the reaming axis forreaming the humeral head 62 in order to create a cavity for theepiphysis portion of the humeral component. It is important to ensurethe epiphysis cavity is correctly reamed, such that once implanted therim of the epiphysis portion is exactly parallel to the resectionsurface. The rim of the epiphysis portion may be required to becongruent with the resection surface. Normally, this requires that theaxis of the neck portion 204 is perpendicular to the resection surface,however the axis of the neck portion 204 may lie anywhere within a planedefined by the axis of the cavity and a line extending from the axis ofthe cavity perpendicular to the resection surface. To ensure that theaxis of the neck portion 204 lies within this plane, the intramedullaryreaming guide 200 must be correctly orientated before being driven intothe bone and locked in position by the ribs 210.

In order correctly orientate the intramedullary reaming guide 200 analignment instrument 212 is provided as illustrated in FIG. 13.Alignment instrument 212 comprises a handle 214 and coupler 216 forcoupling to the intramedullary reaming guide 200. The coupler 216comprises a clamp arranged to engage rib 208 on the intramedullaryreaming guide 200. Neck portion 204 is passed through coupler 216 untilflange 206 comes to rest against the coupler 216. The clamp may then betightened onto rib 208 by turning knob 218, which is coupled to internalrod 220 which in turn couples to the clamp. Knob 218 is turned untilinternal rod 220 is no longer visible. Knob 218 further comprises animpaction surface 218A. Once the intramedullary reaming guide 200 isfully received in alignment instrument 212, the longitudinal axis ofhandle 214 is aligned with the longitudinal axis of the stem component202 while the stem portion 202 is partially received in theintramedullary cavity. Once correctly aligned, an impaction force can beapplied to impaction surface 218A drives the intramedullary reamingguide 200 fully into the cavity.

The alignment instrument 212 further comprises a plane finder 222. Planefinder 222 comprises a plate having a surface which defines a planeforming an angle with respect to the longitudinal axis of the handle 214which is the same as that at which the resection surface intersects thelongitudinal axis of the humerus. Typically, this is 155°. If the neckportion 204 is arranged to be perpendicular to the resection surface,then this is the same angle at which the axis of the neck portion 204intersects the axis of the stem portion 202 of the intramedullaryreaming guide 200.

Referring now to FIG. 14, this illustrates the intramedullary reamingguide 200 coupled to the alignment instrument 212. As can be seen, theplane finder 212 is slidably mounted with respect to the handle 214 suchthat it can be raised and lowered parallel to the longitudinal axis ofhandle 214. The plane finder 222 is formed as a horse shoe such that itcan slide over the intramedullary reaming guide 200. The plane finder222 is further provided with parallel support bars 224, 226 which arearranged to be parallel to the longitudinal axis of the handle. Supportbars 224, 226 are slidably received in holes passing through coupler216, which comprises a mounting bracket. Cross bar 228 prevents theplane finder 222 from being fully removed from the alignment instrument212.

The process of inserting the intramedullary reaming guide 200 into theintramedullary cavity begins with sliding plane finder 222 parallel tothe axis of handle 214 until it is fully extended over theintramedullary reaming guide 200. The stem portion 202 can then beprogressively inserted into the intramedullary cavity until the planefinder 222 contacts the resection surface. The handle 214 (and thus theplane finder 222 and the intramedullary reaming guide 200) can then berotated about the longitudinal bone axis until the plane defined by thesurface of the plane finder 222 is parallel to the resection surface, asshown in FIG. 15. As can be seen in FIG. 15, the ribs 210 have not yetmade contact with the cancellous bone of the humeral head 62 and so thealignment instrument 212 together with the intramedullary reaming guide200 can freely rotate and slide within the intramedullary cavity. Oncecorrectly positioned, the intramedullary reaming guide 200 can be drivenhome by applying an impaction force to impaction surface 218A until theplane finder is brought up against mounting bracket 216 as shown in FIG.16. At this point, ribs 210 engage the cancellous bone surrounding theintramedullary cavity and prevent further rotation of the intramedullaryreaming guide 200.

The flange 206 of intramedullary reaming guide 200 is received within arecess on the underside of mounting bracket 216, and the plane finder222 is similar received within a peripheral recess around the undersideof mounting bracket 216, such that when the plane finder 222 is incontact with both the resection surface and the mounting bracket 216 theintramedullary reaming guide 200 is fully inserted into theintramedullary cavity. The underside of flange 206 is in contact withthe resection surface. The alignment instrument 214 can then bedecoupled from the intramedullary reaming guide 200 by unscrewing knob218 leaving the intramedullary reaming guide 200 in position with neckportion 204 protruding from the resection surface of the humeral head 62as shown in FIG. 17.

As will be appreciated, the alignment of the neck portion 204 isdirectly related to the alignment of the support rod about the axis ofthe cavity during the initial resection step described above. That is,after the initial rotational alignment of the cutting guide assemblyrelative to the patient's forearm, each surgical step performed upon thehumeral head 62 is intended to preserve that original orientation.

It will be appreciated that in alternative embodiments the plane findermay differ. For instance it need not be formed as a horse shoe, and mayinstead be any other shape such as an elongate bar. The only limitationto the shape of the plane finder is that it must be arranged to moverelative to the longitudinal axis of the alignment instrument andarranged to contact the resection surface, such that rotation of thealignment instrument causes the plane finder to rotate until it isparallel to the plane of the resection surface.

After the intramedullary reaming guide 200 has been implanted, thehumeral head is ready for reaming to create a cavity for the epiphysiscomponent. As discussed above, the humeral component may either be asingle integral implant incorporating both the stem component and thehumeral component, or it may be modular in which different size stemcomponents and epiphysis components can be coupled together.Advantageously this allows the epiphysis component to be offset from theposition of the neck portion 204 of the intramedullary reaming guide 200in a posterior direction, which can increase joint mobility.Furthermore, in order to achieve a more secure implantation, it ispreferable to insert the stem component in an anatomic orientationreferenced to the bicipetal groove (as discussed in greater detailbelow). However, the orientation of the epiphysis component may differfrom the anatomic position according to the orientation chosen by thesurgeon when resecting the humeral head, as discussed above.Consequently, the modular humeral implant allows for this variation(i.e., distal offset) between the stem component and the epiphysiscomponent.

As will now be described, an instrument kit for reaming an epiphysiscavity allows for an optional posterior offset of the epiphysiscomponent. Additionally, the diameter of the reamed epiphysis cavity maybe varied. Advantageously, the center of the reamed epiphysis cavity andthe size of the reamed epiphysis cavity may be chosen in order to ensurethe best possible coverage of the resection surface (that is, thelargest epiphysis cavity). FIG. 18 illustrates the resected humeral head62 with the neck portion 204 extending perpendicularly from resectionsurface (only the tip of neck portion 204, flange 206 and referenceformation 208 are visible). Positioned over the neck portion 204 is acentered adapter sleeve 300 which is arranged to ensure that reaming forthe epiphysis cavity is centered about the neck portion 204 of theintramedullary reaming guide 200. The centered adapter sleeve 300comprises a generally cylindrical component, the outer surface of whichcomprises a reaming guide such that a reaming head having a cylindricalbore can be positioned over the adapter sleeve 300, thereby ensuringthat the reaming head is correctly aligned with the humeral head 62.

Adapter sleeve 300 further comprises a bore 302 corresponding to andconfigured to accept the diameter of the neck portion 204. The bore 302extends to a proximal part of the adapter sleeve 300 such that the tipof the neck portion 204 is visible, thus confirming that the adaptersleeve 300 is fully seated on the intramedullary reaming guide 200. At adistal end of adapter sleeve 300 is a collar 304, comprising a grooveshaped to accept the reference formation 208. Consequently, when theadapter sleeve 300 is fully seated on neck portion 204 it is preventedfrom rotating about the neck portion 204.

The adapter sleeve 300 shown in FIG. 18 is a centered adapter sleeve,that is the bore 302 is coincident with the outer surface of the adaptersleeve 300, and thus is suitable for reaming where no posterior offsetof the epiphysis cavity is required. As will be described below, adaptersleeves 300 with varying degrees of posterior offset (that is,non-coaxial bores 302) may be used to achieve a posterior offset of theepiphysis component.

In addition to allowing for variable posterior offset, the instrumentkit further allows for different diameter epiphysis cavities to bereamed using reamers with different size reaming heads. However, beforereaming begins, reaming sizing guides can be used to determine thecorrect size of reaming head. Referring to FIGS. 19 and 20, theserespectively show the same centered adapter sleeve 300 illustrated inFIG. 18 in combination with first and second sizing guides 306, 308. Thesizing guides 306, 308 comprise a sleeve 310, 312 arranged to fit overthe outside of the centered reaming adapter 300 and a disc 314, 316.Discs 314, 316 come to rest against the resection surface so that thesurgeon may view what the diameter of the reamed epiphysis would be if areamer having a reaming head of the same size as the sizing guide discis used. Discs 314, 316 include cut outs so that the resection surfacecan be viewed through, as well as around, the disc. The sizing guide 308shown in FIG. 19 is a smaller, size 1, guide, and the sizing guide shown308 in FIG. 20 is a larger size 2 guide. The disc 314 of the size 1guide 306 may be approximately 38 mm in diameter and the disc 316 of thesize 2 guide may be approximately 42 mm in diameter. The bore of eachsizing guide 306, 308 fitting over the adapter sleeve 300 is the same,thus allowing the adapter sleeves 300 and sizing guides 306, 308 to beinterchanged. The sizing guides 306 may be color coded so that they canbe matched to the same color (and same size) reaming head when reamingis conducted.

As can be seen in FIGS. 19 and 20, both sizing guide 306, 308 extendoutside of the resection surface anteriorly. Consequently, it isapparent that the centered adapter sleeve 300 is not appropriate forthis particular humeral head 62. By providing a posterior offset using aposteriorly eccentric adapter sleeve, improved coverage of the resectionsurface can be obtained, as will now be described.

Referring now to FIG. 21, the centered adapter sleeve 300 shown in FIGS.19 and 20 has been replaced by a new adapter sleeve 318 which presents aposterior offset. The offset adapter sleeve 318 presents generally thesame outer cylindrical shape such that it can receive the same sizingguides 306, 308 shown in FIGS. 19 and 20. However, for offset adaptersleeve 318, the bore 320 which receives neck portion 204 is offset fromthe axis of the adapter sleeve defined by the outer cylindrical surface.Markings on the outside of the offset adapter sleeve 318 ensure that theoffset is positioned in a posterior direction as shown in FIG. 21 (whichillustrates the arrangement for a right humerus).

As with the centered adapter sleeve 300 shown in FIG. 18, the bore 320extends to a proximal part of the reaming adapter 318 such that the tipof the neck portion 204 is visible, thus confirming that the adaptersleeve 300 is fully seated on the intramedullary reaming guide 200. At adistal end of adapter sleeve 318 is a collar 322, comprising a grooveshaped to accept the reference formation 208. Consequently, when theadapter sleeve 300 is fully seated on neck portion 204 it is preventedfrom rotating about the neck portion 204. As will be apparent, thegroove 322 for offset adapter sleeve 318 is exactly the same as forcentered adapter sleeve 300 and in the same relationship with bore 320in order to ensure a correct fit over neck portion 204. However, thegroove 322 is offset from the central axis of the offset adapter sleeve318 such that the groove is defined by two fingers of differingthicknesses.

As noted above, both centered adapter sleeve 300 and offset adaptersleeve 318 are generally cylindrical and have the same exterior diameterto ensure compatibility with the sizing guides 306, 308. The exteriordiameter of the adapter sleeves 300, 318 is larger than the diameter offlange 206 to ensure that even for the offset adapter sleeve 318 theexterior surface of each adapter sleeve extends further outwards thanthe flange 206 and the reference formation 208 in all radial directionsabout the neck portion 204. This ensures that when a reaming head ispassed over the adapter sleeves 300, 318 (or any other adapter sleevewith a different degree of posterior offset), there is no contactbetween the reaming head and the intramedullary reaming guide.

Referring to FIG. 22, the smaller size 1 sizing guide 306 is positionedover offset adapter sleeve 318 to check for coverage of the resectionsurface. Offset reaming guide 318 is color coded the same color as thesize 1 sizing guide 306. Six different epiphyses are available for thefinal implant: first and second size centered epiphyses (correspondingto the size 1 and size 2 sizing guides shown in FIGS. 19 and 20), anepiphysis with a first degree of offset having the diameter of the size1 sizing guide shown in FIG. 22 (in left and right shoulder options) andan epiphysis with a second, larger, degree of offset having the diameterof the size 2 sizing guide shown in FIG. 23 discussed below (also inleft and right shoulder options). Each offset epiphysis has left andright options, which are mirror images of one another.

While a reamer matched to the size of the larger sizing guide could beused on the adapter sleeve shown in FIG. 21 and 22, this would not matcha final implant epiphysis. Consequently, while either size sizing guidemay be used in conjunction with the centered adapter sleeve, thecorrectly color matched sizing guide must be used with each offsetadapter sleeve. If the bone coverage is not sufficient then a newadapter sleeve 324 with an increased posterior offset as shown in FIG.23 can be used in combination with the size 2 sizing guide 308.Increased offset adapter sleeve 324 is color coded the same color as thesize 2 sizing guide 308.

In alternative embodiments of the present invention there may be anynumber of adapter sleeves with differing degrees of offset. Similarly,there may be any number of sizing guides, which may be used with anyadapter sleeve (offset or centered). However, it will be appreciatedthat such flexibility would necessarily be at the expense of having toprovide a larger number of different sized and shaped epiphyses for thefinal implant to account for all possible combinations of size of offsetand size of reaming head (corresponding to the sizing guide).

The color (and hence size) of the chosen sizing guide 306, 308 must bematched to the same color reaming head. Careful note must be taken ofwhether a centered or which posterior offset adapter sleeve is used, andthe size of the reaming head used as this determines which epiphysiscomponent to use during final implantation of the humeral component.

Once the optimal adapter sleeve and sizing guide have been selected thesizing guide is removed and the matching reaming head 326 is passed overthe adapter sleeve such that powered reaming of the epiphysis cavity canbegin as shown in FIG. 24. Reaming head 326 comprises a reaming shell328 including spaced apart reaming formations 330 and an exterior smoothreamer flange 332. Reaming is complete when the exterior reamer flange332 is fully in contact with the resection surface around the whole ofthe reamer shell 328.

Once reaming of the humeral head 62 is complete the reaming head 326 andthe adapter sleeve can be removed from neck portion 204. Theintramedullary reaming guide 200 can then be extracted from theintramedullary cavity by connecting the intramedullary reaming guide 200to the alignment instrument 212 shown in FIG. 13 and pulling axially outof the intramedullary cavity. If any cancellous bone remains unreamedwithin the epiphysis cavity around the previous position of theintramedullary reaming guide 200 then this can be manually removed.

After reaming of the epiphysis cavity is complete, the intramedullarycavity must be enlarged in order to accommodate the stem portion of thehumeral component. As described above, the intramedullary cavity isinitially formed as a continuous diameter reamed bore. At a distalportion, the stem portion comprises a corresponding diameter shaft (arange of diameter stem portions being available corresponding to thelargest size reamer used to create the intramedullary cavity). However,proximally, the stem portion comprises anterior and posterior ribs and,optionally, a pronounced medial rib, all of which serve to preventrotation within the intramedullary cavity and also to increase theengagement of the stem portion with cancellous bone. Therefore, theintramedullary cavity must be enlarged in these areas.

As discussed above, the resection surface, and hence the position of theepiphysis component, can be orientated about the longitudinal axis ofthe bone defined by the intramedullary cavity in order to provide adesired degree of retroversion or anteversion to the reverse shoulderprosthesis. Consequently, the resection surface may be rotationallyoffset from the anatomical position (that is, the rotational position ofthe natural humerus neck axis about the longitudinal axis of thehumerus). However, it is advantageous to insert the stem portion in ananatomical position in order to increase the strength of the joint.Additionally, this provides the maximum amount of cancellous bone forthe stem portion to engage. Therefore, it is necessary to measure therotational offset between the rotational position of the stem portioncavity (that is, the anatomical position of the natural humerus if thestem portion is exactly aligned with the anatomical position) and a lineextending normal to the resection surface and intersecting thelongitudinal axis of the intramedullary cavity. This measurement mayeither be performed at the same time as enlarging the intramedullarycavity or as a separate processing step. The measured rotational offsetmay then be used during assembly of the humeral component torotationally offset the stem portion and the epiphysis portion. It isimportant to correctly measure this offset in order to ensure that therim of the epiphysis portion is parallel to the resection surface.Typically, the rim of the epiphysis component is required to becongruent with the resection surface.

Referring now to FIG. 25, this illustrates a broach 400 and a broachinsertion instrument 402. The broach insertion instrument 402incorporates means for measuring the rotational offset. The broach 400comprises at a distal portion a smooth shaft of a corresponding diameterto that of the intramedullary cavity reamed before. FIG. 25 illustratesthe broach 400 partially inserted into the humeral head 62. At aproximal end the broach 400 comprises cutting teeth 404 adapted toengage and cut into cancellous bone as the broach 400 is driven into thehumeral head 62. Cutting teeth 404 form an anterior cutting fin 406, amedial cutting fin 408 and a posterior cutting fin (not visible in FIG.25).

The enlarged portion of FIG. 25 illustrates from a superior angle theanterior cutting fin 406. To ensure that the broach 400 (and hence, theimplanted stem portion) are in the anatomic position, the anteriorcutting fin 406 should be aligned with the anterior aspect of thebicipital groove 410.

The broach insertion instrument 402 includes an engagement mechanism 412for engaging the distal end of broach 400 that may comprise a clampwhich is engaged by manipulating lever 414. The instrument 402 alsocomprises a handle portion 416, which terminates at an impaction surface(not shown in FIG. 25) to which an impaction force may be applied todrive the broach 400 into the intramedullary cavity.

The instrument further comprises a depth stop 418, which comprises arocker bar extending through a portion of the instrument proximal to thebroach engagement mechanism 412. The rocker bar 418 pivots within theinstrument 402 and extends from the instrument 402 on the anterior andposterior sides. The rocker bar comprises a plane finder. As broach 400is driven into the intramedullary cavity the rocker bar contacts theresection surface at the cortical shell of the humeral head 62 andaligns itself with the plane of the resection surface. In the event thatthe resection surface is oriented in the anatomical position (that is,there is no rotational offset) both arms of the rocker bar 418 willcontact the resection surface at the same time. However, if theresection surface is retroverted or anteverted one or the other arm ofthe rocker bar 418 will contact the resection surface first, causing therocker bar to pivot about its mid point. Insertion of the broach 400into the intramedullary cavity continues until both arms are in contactwith the resection surface. The rocker bar 418 therefore ensures thecorrect extent of insertion of the broach 400 into the intramedullarycavity, and therefore ensures the cavity is correctly sized to receivethe stem portion. Increased rotational offset results in an increasedpivot angle of the rocker bar 418 relative to the broach insertioninstrument 402.

Referring to FIG. 26, this illustrates the broach insertion instrument402 once the broach 400 has been fully inserted into the intramedullarycavity. As can be seen, both arms of rocker bar 418 are in contact withthe resection surface. The rocker bar 418 is pivoted with respect to theinstrument handle 416 due to the resection surface being retrovertedrelative to the anatomical position of the broach 400. The instrument402 further comprises a yoke 420 which is slidably mounted on theinstrument 402 such that it can slide in the plane in which the rockingbar 418 pivots. Yoke 420 comprises legs 422 that are configured tocontact the rocking bar 418. When the rocking bar 418 pivots, it causesthe yoke 420 to rise up away from the rocker bar 418. It will beappreciated that the plane in which yoke 420 slides may differ from thepivot plane of rocker bar 418 and that the yoke 420 will still move solong as its plane is not perpendicular to the rocker bar pivot plane.

If there is no rotational offset (zero retroversion) both legs 422 willbe in contact with the rocker bar 418 and the yoke will not rise up fromits rest position. However, once the rocker bar 418 begins to pivot,only one leg 422 will be in contact with the rocker bar 418. Yoke 420slides within parallel grooves formed in the sides of insertioninstrument 402. The yoke 420 is constrained by these grooves such thatit cannot pivot, the degree to which the yoke 420 rises up is the sameregardless of which arm of rocking bar 418 is rising up.

It will be appreciated that an increased rotational offset will causethe rocker bar 418 to pivot by an increased amount. The direction inwhich rocker bar 418 pivots (that is, which arm is uppermost) isdependent upon whether the resection surface is retroverted oranteverted. The amount by which yoke 420 rises up when rocker bar 418pivots it directly proportional to the magnitude of the of the rockerbar pivot, and hence is indicative of the rotational offset between thestem portion and the epiphysis portion. The enlarged portion of FIG. 26illustrates a scale 424 applied to either side of the yoke 420. Scale424 is read at a reference mark 426 on the body of the broach insertiontool. When there is no rotational offset (rocker bar 418 is level andthe yoke 420 is at its lowest position) the reference mark 426 indicates0° retroversion. Scale 424 is calibrated to directly indicate therotational offset. Therefore, if the resection surface is not in theanatomical position reading scale 424 will indicate the degree ofrotational offset (in FIG. 26 the scale 424 indicates up to a 30°rotational offset). Whether the rotational offset is in respect ofretroversion or anteversion is determined by noting which arm isuppermost (and in contact with yoke 420). If the anterior arm of rockerbar 418 is uppermost, the resection surface is retroverted and if theposterior arm is uppermost, the resection surface is anteverted.

It will be appreciated that in alternative embodiments of the presentinvention the broach insertion instrument may comprise alternative meansfor measuring movement of the yoke (or other plane finder component)such as electronic detection means.

As will be appreciated, it is advantageous for the rocker bar 418 to beas long as possible, and for the yoke legs 422 to contact the rocker bar418 as far apart as possible as this amplifies the degree to which theyoke 420 rises up for a given rotational offset. Typically, the rockerbar is 54 mm, though it will be appreciated that the length of therocker bar must be greater than the diameter of the cavity reamed in theresection surface of the epiphysis. The proximal place of the implantedepiphysis has a diameter which typically ranges between 38 mm and 41 mmaccording to the required size of the implant. Therefore, the rocker barmay vary between 40 mm and 70 mm. The yoke legs 422 are arranged tocontact the rocker bar 418 towards either end of the rocker bar 418, forinstance spaced apart by between 40 mm and 70 mm.

It will be appreciated that other mechanisms for measuring therotational offset could be provided. The rocker bar 418 constitutes aplane finder adapted to alter its position relative to the broachinsertion tool to conform to the plane of the resection surface.Specifically, the movement is a pivot motion about an axis which isperpendicular to the axis of the broach insertion instrument handle 416.The movement relative to the handle 416 could take other forms. Forinstance, the plane finder may be formed as a plate having a plane whichintersects the axis of the handle 416 at the same angle as that at whichthe resection surface intersects the longitudinal axis of theintramedullary canal. The plane finder may be arranged to be rotatableabout the handle 416 such that as the broach 400 is driven into the bonethe plane finder slides round until its plane is congruent with theresection surface. The rotation of the plane finder about the handle 416may then be measured and is equivalent to the rotational offset. Asnoted above, the reaming and measurement steps may be separated suchthat a separate measurement tool could be used have a first componentfor insertion into the intramedullary cavity and a plane finder asdiscussed above.

Referring now to FIG. 27, this illustrates the humeral component forinsertion into the reamed humeral head 62. Alternatively, the componentillustrated in FIG. 27 may be a trial component for insertion andtesting prior to insertion of the final component. FIG. 27 illustrates aproximal part of the stem portion 428 and the epiphysis portion 430. Thestem portion 428 further comprises a reference formation 432 and theunderside of the epiphysis 430 comprises a series of notches 434 and ascale 436. An upper surface of the stem 428 further comprises a singleprotrusion (not visible in FIG. 27) arranged to engage one of notches434. The measured rotational offset of the enlarged intramedullarycavity and the epiphysis cavity (measured using the yoke system of thebroach insertion instrument 402 of FIGS. 25 and 26) according to whetherthe offset is retroverted or anteverted, is used to select theappropriate notch 434 into which to engage the stem protrusion, asselected by aligning reference formation 432 with the required positionon scale 436. A locking screw (not visible in FIG. 27) passes throughthe epiphysis portion into the stem portion, thereby locking the humeralcomponent and preserving the selected degree of retroversion oranteversion such that the humeral component conforms to the shape of thecavity reamed in the humeral head 62. The stem portion 428 is selectedaccording to the diameter of the reamed intramedullary cavity. Theepiphysis portion 430 is selected from an available range according towhether the reamed epiphysis cavity was centered or posteriorly offset,and according to the size of the epiphysis reaming head used.

Once assembled, the humeral implant can be manipulated using a humeralcomponent driver which comprises means for releasably engaging theinside part of the epiphysis component. This allows the humeralcomponent to be inserted into the intramedullary cavity withoutcontacting the exterior surface of the implant (thereby preserving thehydroxyapetite coating which serves to encourage bone in growth securingthe implant in position). The humeral component driver incorporatesalignment holes to receive an alignment pin similar to the alignment pinshown in FIG. 3 allowing the alignment of the humeral component to bereferenced to the patient's forearm, thereby ensuring that the humeralcomponent is aligned with the resection surface. Furthermore, duringinsertion of the humeral component, the anterior rib of the stemcomponent is aligned with the anterior aspect of the bicipetal groovesimilar to the alignment of the broach shown in FIG. 25.

As noted above, in place of the modular humeral component an integralhumeral implant comprising both a stem and an epiphysis may be used. Theintegral humeral component is particularly suited to applications inwhich the humeral component is secured using bone cement. The surgicalsteps for preparing the intramedullary cavity to receive the integralimplant are generally the same as described above for the modularimplant. However, it is not possible to provide a posterior offset forthe epiphysis. Consequently, only a single, centered reaming adapter isprovided for reaming the epiphysis cavity, although a choice of size ofreaming head is available and reaming sizing guides may be used todetermine the reaming head to be used, as described above. It is notnecessary to enlarge the intramedullary cavity using a broach to receivean integral component as fixation is achieved using bone cement andtherefore the humeral component stem does not incorporate fins. Duringinsertion of the humeral component into the intramedullary cavityrotational alignment of the component and the resection surface isachieved by using an alignment pin orientated to be parallel to thepatient's forearm axis, as described above for the modular humeralimplant.

Once the humeral implant is in position, the convex bearing head can beattached to the mounting plate. As with the humeral component, a trialconvex bearing head may first be attached so that the optimalpositioning and size of the convex bearing head can be determined. Theconvex bearing head comprises a convex dome including a recessed cavityon the reverse side corresponding to the size and shape of the mountingplate. As is shown generally in FIG. 30 and described in more detailbelow, a hole passes through a central portion of the convex bearinghead such that the convex bearing head can be secured to the mountingplate by passing a screw into a threaded socket extending into thecentral pin of the mounting plate. At its broadest point the convexbearing head is preferably either approximately 38 mm or 42 mm indiameter according to the chosen size, but the size may range from 35 mmto 45 mm. Additionally, the convex bearing head may be circular oreccentric about the screw hole. The convex bearing head preferablyoverlaps the glenoid inferior limit by about 3 mm to 5 mm. The overlapreduces contact between the humeral epiphysis and the scapular pillarduring rotation of the shoulder.

When securing the convex bearing head to the mounting plate a 1.5 mmdiameter guide pin may be inserted into the central hole in the mountingplate in order to ensure correct alignment of the convex bearing head. Afixation hole within the convex bearing head is passed over the guidepin until the recessed cavity on the reverse side of the convex bearinghead is in contact with the mounting plate. A fixing screw includes anaxial bore configured so as to permit the fixing screw to pass over theguide pin. The fixing screw can be tightened using a cannulatedhexagonal screw driver. Once the fixing screw is engaged in the threadedbore within the mounting plate central pin the guide pin can be removedbefore fully tightening the screw. The screw is preferably tighteneduntil the scapula begins to rotate in response to motion of the screwdriver.

For an eccentric convex bearing head, it is important that theeccentricity is in the correct radial position. The maximum eccentricityshould be directed towards the base of the glenoid. Referring to FIG.28, in order to rotate the convex bearing head upon the mounting platethe convex surface of the convex bearing head 500 incorporates areference formation 502. The reference formation 502 can be manipulatedby a convex bearing head orientation guide 504, which is arranged toslide over the screw driver. As shown in FIG. 28, the referenceformation 502 comprises an eccentric slot enlarging part of the fixationhole 506. The fixation hole 506 including slot 502 is shown more clearlyin a front view of the convex bearing head 500 in FIG. 31. The convexbearing head orientation guide 504 includes a pin 508 which protrudesfrom the body of the guide 504 and is configured to be received withinthe slot 502.

The orientation guide 504 may further comprise a circular guide arrangedto be received within the fixation hole 506. The screw driver is passedthrough the guide 504 and engages the fixing screw within fixation hole506. The convex bearing head 500 can be rotated about the fixation hole506 by manipulating the body of the convex bearing head orientationguide 504 while tightening the fixing screw.

Referring to FIG. 29, the outside surface of the convex bearing headorientation guide 504 preferably includes an arrow 5 10. The correctrotation position of the convex bearing head 500 can be achieved byrotating the convex bearing head orientation guide 504 (and hence theconvex bearing head 500) until the arrow 510 is aligned with apredetermined part of the scapula 512. For instance, if the requiredradial position of maximum eccentricity is towards the inferior portionof the glenoid, the reference formation 502 (which is aligned with themaximum eccentricity) should also point to the inferior portion of theglenoid. The arrow 510 is on the opposite side of the orientation guide504 from the pin 508 and should be aligned with the superior portion ofthe glenoid. In particular, the convex bearing head 500 and theorientation guide 504 should be rotated until the arrow 510 points tothe base of the coracoid process in order to correctly align the convexbearing head before tightening the fixing screw while maintaining theguide 504 in position. The convex bearing head may be further secured tothe mounting plate by applying an impaction force to the convex bearinghead and then further tightening the fixing screw.

It will be apparent to the skilled person that in alternativeembodiments the reference formation may differ from that illustrated inFIG. 28, for instance it could be a different shape or may not be formedtogether with the fixation hole.

Referring to FIG. 30, this illustrates a cross section view of theorientation guide 504 engaged with the convex bearing head 500. Guidepin 508 is received within slot 502 within the fixation hole 506. Ascrew driver 514 is shown extending through an axial lumen within theorientation guide 504 to engage fixing screw 516 which secures theconvex bearing head 500 to the mounting plate 518. In alternativeembodiments in which the reference formation is not combined with thefixation hole, the alignment guide and the screwdriver may be providedseparately.

The reverse shoulder prosthesis is completed by positioning a cup in arecess in the upper surface of the epiphysis. The cup presents a concavebearing surface in which the convex bearing head is received. The sizeof the cup (for example, 38 mm or 42 mm in diameter) is chosen to matchthe size of the convex bearing head. Additionally, the cup is availablein a range of thicknesses. The cup thickness chosen is dependent uponthe precise positioning of the resection surface and the mounting plate.If the implanted prosthesis results in an insufficiently tensionedshoulder joint (in which the joint tends to dislocate during motion)then a thicker cup may be used to increase the tension by increasing thedistance between the scapula and the humerus.

It can be necessary to change the humeral component to the anatomicconfiguration (and also to change the glenoid component to the anatomicconfiguration). This may either be during a revision procedure due toglenoid loosening or during the initial surgical procedure to implantthe prosthesis if it becomes apparent that there is insufficient glenoidbone stock to attach the mounting plate after the point at which thehumerus has been resected.

In order to change the humeral component to the anatomic configurationit is necessary to remove cortical bone in the medial and lateralregions around the humeral head. This is because the anatomic head to befitted to the implanted humeral implant overlaps the cortical bone inthese regions. It is important to minimize any disturbance the humeralstem during this bone preparation stage.

The first step is to remove the humeral cup from the epiphysis. Areaming guide 600 can then be inserted into the cavity within theepiphysis 602 as shown in FIG. 32 and connected to the existingepiphysis 602 by a taper junction. That is, the epiphysis 602 comprisesa shallow cavity formed generally as a portion of a cone. The sides ofthe open cavity formed in the epiphysis 602 diverge towards the openend. Similarly, the reaming guide 600 generally comprises a disc havingtapering edges arranged to match the taper of the epiphysis open cavity.The taper junction forms a firm connection between the epiphysis 602 andthe reaming guide 600 during reaming of the bone, while allowing thereaming guide 600 to removed later. The reaming guide 600 comprises twosockets 604 that define reaming axes. The reaming axes diverge as theyextend from the epiphysis 602. The reaming guide 600 is positioned inthe epiphysis 602 such that anterior and posterior slots 606 are alignedwith corresponding slots 608 in the rim of the epiphysis 602. Thereaming guide 600 forms a press fit with the epiphysis component cuptemporarily securing guide 600 in position. The reaming axes aredirected medially and laterally.

As shown in FIG. 33, an appropriate reaming head 610 is driven about anaxial guide 615 that is arranged to be inserted into socket 604. Thereaming head 610 is driven by a means (not shown) to remove corticalbone in the lateral (or medial, depending on the chosen axis) directionwithout contacting the implanted stem component. The reaming head 610comprises a reaming ring 612 that is configured to pass around theepiphysis 602 contacting the cortical bone immediately surrounding theepiphysis 602 predominantly in the medial (or lateral) direction.Reaming ring 612 is driven radially about axial guide 615 to remove boneon the proximal portion of the humerus.

As shown in FIG. 34, once bone is removed on the medial and/or lateralportions of the humerus, an appropriately sized humeral head implant 614is secured to the epiphysis forming a press fit in the epiphysis cavityoverlapping the epiphysis and the cortical bone in the medial andlateral directions.

Although surgical instruments and techniques described above areprimarily related to a reverse shoulder prosthesis implantationprocedure it will be appreciated that some or all of the surgicalinstruments and surgical techniques described may be equally applicableelsewhere. For instance, they may find utility in the implantation ofother prostheses, such as a hip prosthesis. Additionally, some or all ofthe surgical instruments and techniques described may be equallyapplicable to the implantation of anatomic prostheses as opposed toreversed anatomy prostheses.

Other modifications and applications of the present invention will bereadily apparent from the description herein without departing from thescope of the appended claims.

1. An instrument for measuring an angle about a cavity axis defined by acavity extending into a head of a bone, the bone having been resected ona plane that is inclined relative to the cavity axis, the instrumentcomprising: a stem arranged to be inserted into the cavity; a planefinder coupled to the stem arranged to contact the resection surface asthe stem is inserted into the cavity and to move relative to the stemuntil the plane finder lies in a plane parallel to the resectionsurface, movement of the plane finder relative to the stem beingindicative of the angle about the cavity axis between a radial positionindicated by part of the stem and a line extending perpendicularly fromthe resection surface; and a scale configured such that movement of theplane finder relative to the stem is indicated by the position of aportion of the plane finder relative to the scale.
 2. The instrument ofclaim 1, wherein the plane finder is coupled to the stem via a handleportion, the angle about the cavity axis being indicated by movement ofthe plane finder relative to the handle.
 3. The instrument of claim 1,wherein the stem comprises a broach configured to form an eccentricenlarged portion of the cavity at a proximal portion of the cavity. 4.The instrument of claim 3, wherein the plane finder is coupled to thestem via a handle portion, the handle further comprising an impactionsurface arranged such that an impaction force applied to the impactionsurface is transmitted to the broach to drive the broach into thecavity.
 5. The instrument of claim 3, wherein the broach comprisesanterior and posterior fins for engaging the sidewalls of the cavity anda medial fin for forming the eccentric enlarged portion at a medial sideof the proximal end of the cavity.
 6. The instrument of claim 5, whereinthe radial position indicated by the stem comprises the anterior fin. 7.The instrument of claim 1, wherein the stem further comprises aneccentric protrusion arranged to locate within an eccentric enlargedportion of the cavity at a proximal portion of the cavity to determinethe rotational position of the stem within the cavity.
 8. The instrumentof claim 1, wherein the plane finder comprises an elongate rocker barpivotably coupled to the stem such that the rocker bar pivots about apivot axis extending transverse to the axis of the stem.
 9. Theinstrument of claim 8, wherein the rocker bar is configured to pivotabout its pivot axis until it contacts the resection surface on eitherside of the pivot axis.
 10. The instrument of claim 9, wherein therocker bar is configured to prevent further insertion of the stem intothe cavity when the rocker bar contacts the resection surface on eitherside of the pivot axis.
 11. The instrument of claim 8, wherein the pivotaxis lies in a plane that, in use, is parallel to the resection surface,the pivot axis intersecting both the axis of the stem and a lineextending from the cavity axis perpendicular to the resection surface.12. The instrument of claim 8, further comprising a yoke coupled to thestem such that the yoke is slidable in the pivot plane of the rockerbar, the yoke comprising a pair of arms, each arm extending towards therocker bar on opposite sides of the pivot point, wherein pivoting of therocker bar causes the rocker bar to contact an arm of the yoke causingthe yoke to slide away from the rocker bar.
 13. The instrument of claim12, wherein movement of the plane finder relative to the stem causes theyoke to slide, the yoke translating movement of the plane finder to thescale such that movement of the plane finder relative to the stem isindicated by the position of a portion of the yoke relative to thescale.
 14. The instrument of claim 13, wherein the scale is arrangedsuch that movement of the yoke can be measured by comparing the positionof a fixed point on the yoke to the scale.
 15. The instrument of claim1, wherein the plane finder comprises a plate coupled to the stem, theplate including a surface defined by a plane which intersects the axisof the stem at the same angle as that at which the resection surface ofthe bone intersects the cavity axis, the plate being configured torotate about the axis of the stem until the surface of the plate lies ina plane parallel to the resection surface.
 16. The instrument of claim15, wherein, when the surface of the plate lies in a plane parallel tothe resection surface and is in contact with the resection surface,further insertion of the stem into the cavity is prevented.
 17. A methodof measuring an angle about a longitudinal axis defined by a cavityextending into a head of a bone, the bone having been resected on aplane which is inclined to the longitudinal cavity axis, the methodcomprising: inserting a stem into the cavity; coupling a plane finder tothe stem, the plane finder contacting the resection surface as the stemis inserted into the cavity, the plane finder moving relative to thestem until the plane finder lies in a plane parallel to the resectionsurface; and measuring movement of the plane finder relative to the stemat a scale coupled to the stem, said measured movement being indicativeof the angle about the longitudinal cavity axis between a radialposition indicated by part of the stem and a line extendingperpendicularly from the resection surface.
 18. The method of claim 17,wherein the stem comprises a broach and the plane finder is coupled tothe stem via a handle portion, the handle further comprising animpaction surface, said step of inserting the stem into the cavityfurther comprising applying an impaction force to the impaction surfacesuch that the impaction force is transmitted to the broach driving thebroach into the cavity forming a medial enlarged portion of the cavityat a proximal portion of the cavity.
 19. The method of claim 18, whereinthe plane finder comprises an elongate rocker bar pivotably coupled tothe stem, said step of measuring movement of the plane finder relativeto the stem further comprising measuring the pivoting of the rocker barabout a pivot axis extending transverse to the axis of the stem when therocker bar contacts the resection surface on either side of the pivotaxis preventing further insertion of the stem into the cavity.
 20. Themethod of claim 18, wherein said step of inserting the stem into thecavity further comprises inserting the broach into a cavity in the endof a humerus such that the broach is orientated about the longitudinalcavity axis in an anatomic position by aligning an anterior rib with theanterior aspect of the bicipital groove.